Measurements of Neutral Density in the Alcator C-Mod Tokamak

In magnetically confined plasmas, the study of edge dynamics has many important implications which include overall confinement and performance, and, in particular, calls for a quantitative understanding of the particle inventory and its sources. In fact, when studying the particle, momentum and energy balance of the edge plasma, one must include the contribution of neutral particles. However, direct measurements of the neutral population, i.e. neutral density and associated ionization rate, are particularly arduous in that region. Based on a simple measurement of the excitation of neutral deuterium (and hydrogen), we installed a series of diagnostics in the Alcator C-Mod tokamak, from which we can obtain those two quantities with high radial resolution. In addition we installed a series of modified Penning- type gauges to measure the neutral pressure in close proximity of the discharge itself.